In one aspect, the present invention relates to a charging device suitable for use with a tubing system. In another aspect, the present invention relates to a method for precharging a tubing system to facilitate further assembly of a unit which contains the tubing system.
One type of subsea wellhead used in the oil and gas industry is the horizontal, or side valve tree (SVT). The SVT is formed from a spool body having a through passage for tubing placement and a side outlet for production. A tubing hanger having a through passage and a side outlet is positioned in the spool body, with the side outlet of the tubing hanger positioned in flow communication with the side outlet in the spool body, usually in alignment. The production tubing string extends downwardly from the tubing hanger, and the through passage through the tubing hanger is sealed above the side outlet of the tubing hanger, so that production fluids will flow up the production string and then turn to flow through the side outlets of the tubing hanger and the spool body.
A downhole safety valve is carried by the production tubing. This valve is hydraulically actuated. One of the challenges in side valve tree design is in providing hydraulic connectivity between the downhole safety valve and the rig or platform on the surface across an interface situated in the SVT.
A highly suitable assemblage for providing such hydraulic connectivity is disclosed in U.S. Pat. No. 5,582,438 to Wilkins et al. issued Dec. 10, 1996, the disclosure of which is incorporated herein by reference. This assembly relies on a pair of coupling elements, one carried in the spool body, the other by the tubing hanger assembly to establish the hydraulic flow passage when the tubing hanger assembly seats in the spool body. A difficulty, however, is sometimes presented in operation of this device, as well as other similar devices, as the tubing hanger assembly is lowered downhole. The problem is that the downhole safety valve, which is carried in the production tubing string, is designed to fail in the closed position in the event that its supply of hydraulic fluid pressure is interrupted. The valve being in the closed position as the tubing string is lowered downhole prevents fluid flow through the tubing string, and can cause problems in seating the tubing hanger in the spool body.
A method for maintaining the downhole safety valve in the open position as the tubing string is lowered into the well would be very desirable.
An apparatus which is well suited for causing the downhole safety valve to stay in the open position as the tubing string is lowered into the well would be very desirable.